In recent years devices using Doppler radar are used as measurement devices to measure the speed of travel, rate of rotation (amount of spin), and so on of balls for ball games.
In these devices, a transmission wave that includes microwaves is sent towards the ball for a ball game from an antenna, and the reflection wave reflected from the ball for a ball game is measured, and the speed of travel and the rate of rotation is obtained based on the Doppler signal obtained from the transmission wave and the reflection wave.
In these cases, the reflection wave must be obtained efficiently in order for the speed of travel and the rotation to be measured stably and reliably. In other words, efficiently obtaining the reflection wave is beneficial in the securing of measuring distance.
On the other hand, technology has been suggested for providing a layer or film including a metallic material throughout an entirety of a surface of a ball in order to enhance visual appearance and/or design (see Japanese Unexamined Patent Application Publication Nos. 2007-021204A, 2004-166719A and 2007-175492A).
Additionally, technology has been suggested for providing a metallic layer having a spherical surface shape between a core layer and a cover of a ball in order to ensure reaction (see Japanese Unexamined Patent Application No. H11-076458A).
According to tests carried out by the inventors of the present technology, it was found that although forming a layer or film that includes a metal material uniformly on the spherical surface of a ball is beneficial in terms of ensuring the radio wave reflection properties, the reflection wave tends to be reflected by the layer or film over only a comparatively narrow range by specular reflection of the transmission wave, so this is disadvantageous for receiving the reflection wave by the antenna.
As a result, insufficient measurement distance was provided for determining the speed of travel, the trajectory, and the rate of rotation which represent the behavior of the ball for a ball game.